The present invention relates to a speech coding and more particularly to the speech coder and coding method using harmonic-noise speech coding algorithm capable of achieving more improved speech quality by using cepstrum analysis method and LPC (Linear Prediction Coefficient) analysis method for the mixed signal of voiced/unvoiced sound which is not represented well in the generally used harmonic coding algorithm.
As harmonic model is generally based on sinusoidal analysis and synthesis in the low rate speech coder, noise component with non-stagnant characteristic is not represented well. Therefore, the method for modeling noise component observed in the spectrum of real speech signal has been required.
In order to meet these demands, the research for the harmonic speech coding model such as MELP (Mixed Excitation Linear Prediction) algorithm or MBE (Mixed Band Excitation) algorithm which are known as guaranteeing good speech quality, has been progressed, in which the characteristic of said algorithms is that speech can be observed by dividing the speech into each band and then analyzing it.
However, said algorithms analyze with fixed bandwidth the sound in which voiced/unvoiced sound signal is multiply mixed. And due to the binary decision structure, which is deciding voiced/unvoiced sound at each band, also have limitation on effective representation. And particularly, in the case that voiced/unvoiced sounds are mixed simultaneously or the mixed signal is distributed on the band border, there is a disadvantage that the spectral distortion is occurred.
These disadvantages are caused by using the signal modeling method utilizing only the frequency peak value of the harmonic model in the mixed signal of voiced/unvoiced sound. These cases are caused by insufficient representation of the mixed signal of voiced/unvoiced sound of the low rate model. Recently, in order to solve these disadvantages, the studies on the coding methods for the mixed signal of voiced/unvoiced sound have been actively progressed.
The object of coding for mixed signal of voiced/unvoiced sound is to represent effectively voiced sound spectral part and unvoiced sound spectral part in frequency domain. And there are two coding methods in recent analysis method. The first coding method is dividing into two parts of voiced/unvoiced bands after defining frequency transitional point and the second coding method is differing mixing level of voice/unvoiced sound during synthesis after defining probability value of voiced sound from total spectral information.
As an example of the second coding method, there is U.S. Pat. No. 5,774,837 entitled xe2x80x9cSpeech Coding System And Method Using Voicing Probability Determinationxe2x80x9d written by Suat Yeldener and Joseph Gerard Aguilar. The patent described the technology which, in order to analyze and synthesize the mixed signal of voiced/unvoiced sound, analyzes the spectral of voiced sound and modified linear prediction parameter of unvoiced sound and by using the analyzed results synthesizes the mixed signal according to the degree of the probability value of the voiced sound computed from the pitch and parameter which are extracted from the spectrum of the inputted speech signal.
However, above mentioned prior art or technologies extract unvoiced sound by dividing spectral of the mixed signal of voiced/unvoiced sound into two sections and as the analysis and synthesis of the in putted speech signal are based on the probability value, it is impossible to do sound analysis and synthesis effectively through real spectral values of all sections.
According to a first aspect of the present invention, a harmonic-noise speech coder of the mixed signal of voiced/unvoiced sound using harmonic model is provided. The harmonic-noise coder comprises a noise spectral estimating means for coding the noise component by predicting the spectral by LPC analysis method after separating the noise component which is unvoiced sound component from the inputted LPC residual signal using cepstrum.
Also, according to a second aspect of the present invention, a harmonic-noise speech coding method of the mixed signal of voiced/unvoiced sound includes the following step: A harmonic coding step for coding voiced sound out of the mixed signal; And noise coding step for coding unvoiced sound out of the mixed signal. Preferably, the noise coding step is composed of a cepstrum analyzing step for extracting noise spectral envelope by cepstrum analyzing the mixed signal and an LPC analyzing step for extracting noise spectral envelope information from the extracted spectrum.